Method and apparatus for time synchronization in a communication system

In a cable modem communications system comprising a headend unit that includes a master clock and plurality of cable modems coupled of the headend unit wherein each cable modem includes a local clock, a method for synchronizing the local clock of at least one of the cable modems, the method comprising the steps of:

• receiving a first timestamp with the cable modem, wherein the first timestamp comprises a signal generated by the headend unit in response to the value of the master clock at the time the first timestamp signal is generated;
• generating a first cable modem time reference with the cable modem in response to the receipt of the first timestamp;
• receiving a second timestamp with the cable modem, wherein the second timestamp comprises a signal generated by the headend unit in response to the value of the master clock at the time the second timestamp signal is generated;
• generating a second cable modem time reference with the cable modem in response to the receipt of the second timestamp;
• generating a headend difference time comprising the difference of the first and second timestamps;
• generating a cable modem difference time comprising the difference of the first and second cable modem time references;
• generating a clock error time that comprises the difference of the headend difference time and the cable modem difference time;
• generating a correction factor in response to the clock error time;
• providing the correction factor to an accumulator device;
• and
• generating a synchronized downstream reference signal by adjusting an output clock signal of the local clock in response to the accumulator device.

In a communications system comprising a headend unit including a master clock and a plurality of cable modems coupled to the headend unit, a cable modem apparatus comprising:

• an input/output node coupled to the headend unit;
• a receiver, coupled to the input/output node, that receives a first and second timestamp generated by the headend unit in response to the value of the master clock at the time the first and second timestamp signals are generated, respectively;
• a local clock;
• and
• a synchronization counter, coupled to the receiver, that generates a first and second cable modem time reference in response to the receipt of the first and second timestamp, respectively, wherein the synchronization counter further includes:
• a correction signal generator that generates a correction factor in response to the first and second timestamp and the first and second cable modem time reference;
• and
• a correction circuit, including an accumulator device responsive to the correction factor, wherein the correction circuit generates a synchronized downstream reference signal by adjusting an output clock signal of the local clock in response to the accumulator device.

In a communications system comprising a headend unit including a master clock and a plurality of cable modems coupled to the headend unit, a cable modem apparatus comprising:

• an input/output node coupled to the headend unit;
• a receiver, coupled to the input/output node, that receives a first timestamp generated by the headend unit in response to the value of the master clock at the time the first timestamp signal is generated, and that receives a second timestamp generated by the headend unit in response to the value of the master clock at the time the second timestamp signal is generated;
• a local clock;
• and
• a synchronization counter, coupled to the receiver, that generates a first cable modem time reference in response to the receipt of the first timestamp, and that generates a second cable modem time reference in response to the receipt of the second timestamp, wherein the synchronization counter further includes:
• a correction signal generator that generates a correction factor in response to the difference of a headend difference time and a cable modem difference time, wherein the headend difference time comprises the difference of the first and second timestamps, and the cable modem difference time comprises the difference of the first and second cable modem time references;
• and
• a correction circuit, including an accumulator device responsive to the correction factor, wherein the correction circuit generates a synchronized downstream reference signal by adjusting an output signal of the local clock in response to the accumulator device.

In a communications system comprising a control unit that includes a master clock and a plurality of transceiver devices coupled to the control unit wherein each transceiver device includes a local clock, an apparatus for synchronizing the local clock of at least one of the transceiver devices, the apparatus comprising:

• a means for receiving a first and second timestamp, wherein the first and second timestamp comprise signals generated by the control unit in response to the value of the master clock at the time the first and second timestamp are generated, respectively;
• a means for generating a first and second transceiver time reference in response to the receipt of the first and second timestamp, respectively;
• a means for generating a correction factor in response to the first and second timestamp and the first and second transceiver time reference;
• a means for providing the correction factor to an accumulator device;
• and
• a means for generating a synchronized downstream reference signal that comprises an output clock signal of the local clock adjusted in response to the accumulator device.

In a communications system comprising (i) a control unit that includes a master clock and a headend unit and (ii) a plurality of transceiver devices coupled to the control unit wherein each transceiver device includes a local clock and a cable modem, a method for synchronizing the local clock of at least one of the transceiver devices with the master clock, the method comprising the steps of:

• receiving a first and second timestamp with the transceiver, wherein the first and second timestamp comprise signals generated by the control unit in response to the value of the master clock at the time the first and second timestamp signals are generated, respectively;
• generating a first transceiver time reference and a second transceiver time reference with the transceiver device in response to the receipt of the first and second timestamp signals, respectively;
• generating a correction factor in response to the first and second timestamp and the first and second transceiver time reference;
• providing the correction factor to an accumulator device;
• and
• generating a synchronized downstream reference signal by adjusting an output clock signal of the local clock in response to the accumulator device.